This proposal will compare the effectiveness of customary Fit and Strong!-an evidence-based physical activity/behavior change program for older adults with osteoarthritis (OA)-to an enhanced version of the program that also addresses diet and weight. OA is the primary cause of disability among older adults (Hootman et al., 2009), and lower extremity (LE) OA is the pathway through which this disability develops (Dunlop, Hughes, et al., 1998). Fit and Strong! targets older adults with LE OA. In a recent randomized trial, the 8-week Fit and Strong! program demonstrated significant benefits on physical activity maintenance at 18 months accompanied by improved LE stiffness, pain, function, strength, and gait speed as well as reduced anxiety and depression (Hughes et al., 2010). However, Fit and Strong! does not currently address diet/ weight issues present among program participants. This is an important omission because LE symptoms related to OA are either caused by or exacerbated by obesity. Data show that a weight loss of 5.1 kg over a 10-year period decreases the odds of developing knee OA by more than 50% (Felson 1992). In 2007, 37% of men and 48% of women 60 years of age and older were obese (Flegal et al., 2010). Between 2003-2009 obesity prevalence, on average, was 54% higher among adults with arthritis compared to those without the condition (Hootman et al., 2011a). Obese persons with arthritis were also 44% more likely to be physically inactive (Hootman et al., 2011b). This rise in obesity prevalence may explain rapid increases in the use of hip and knee replacement surgery in the U.S., which totaled $9.1 billion in 2004 (Kim, 2008). Importantly, a recent meta- analysis has documented a dearth of evidence supporting the efficacy of interventions to decrease weight in older adults (Witham and Avenell, 2010). We will address this serious scientific gap by developing and testing Fit and Strong! Plus-an enhanced version of Fit and Strong! that addresses both physical activity and diet. We will incorporate common elements from the ADAPT and ORBIT trials that addressed physical activity and diet/ weight loss. Both trials achieved weight loss of approximately 5% at 18 months (Messier et al., 2004, Fitzgibbon et al., 2010). We will compare the effectiveness of customary Fit and Strong! to Fit and Strong! Plus over 24-months using an RCT with 400 participants at 3 community sites. Both groups will receive tapered telephone reinforcement to support maintenance of behavior change during months 3- 24. We will also examine health care use before and after the interventions. We hypothesize that Fit and Strong! Plus participants will show differential, significant improvement in diet behaviors at 2, 6, 12, 18 and 24 months accompanied by significant weight loss (e 5%) at 6 months that will be maintained at 24 months. Fit and Strong! Plus participants will also show differential statisticall significant improvements in physical activity maintenance accompanied by improved LE pain, stiffness, and function as well as depression and anxiety at all time points.